Exploration, and in particular space exploration, is an intrinsically risky and expensive proposition, therefore there is great incentive to look for ways to achieve exploration safely and affordably. Typically it requires about $10,000 per pound to launch mass into orbit. Therefore, there is great interest in finding innovative ways to reduce mass of objects to be sent into orbit. Weight savings may be realized by using more damage tolerant systems to avoid carrying extra weight for replacement sections and repair components. Self-healing materials provide for improved damage tolerance in load bearing structures, and a means of self-mitigation, or self-reliability for overall vehicle health and durability. The self-healing phenomaterials capable of puncture healing upon impact show great promise for space exploration applications wherein an internal structural breach caused by micrometeoroid impacts, which could be catastrophic for the astronaut crew, would be self-contained to allow the retention of air pressure to the greatest extent possible. This approach is also applicable to other pressure vessel type structures which may have critical requirements with regard to a contained fluid (e.g. fuel tanks).
Self-healing materials display the unique ability to mitigate incipient damage and have built-in capability to substantially recover structural load transferring ability after damage. In recent years, researchers have studied different “self-healing mechanisms” in materials as a collection of irreversible thermodynamic paths where the path sequences ultimately lead to crack closure or resealing. Crack repair in polymers using thermal and solvent processes, where the healing process is triggered with heating, or with a solvent, have been studied. A second approach involves the autonomic healing concept, where healing is accomplished by dispersing a microencapsulated healing agent and a catalytic chemical trigger within an epoxy resin to repair or bond crack faces and mitigate further crack propagation. Another related approach, the microvascular concept, utilizes brittle hollow glass fibers (in contrast to microcapsules) filled with epoxy hardener and uncured resins in alternating layers, with fluorescent dye. An approaching crack ruptures the hollow glass fibers, releasing healing agent into the crack plane through capillary action. A third approach utilizes a polymer that can reversibly re-establish its broken bonds at the molecular level by either thermal activation (i.e., based on Diels-Alder rebonding), or ultraviolet light. A fourth approach utilizes structurally dynamic polymers, which are materials that produce macroscopic responses from a change in the materials molecular architecture without heat or pressure. A fifth approach involves integrating self-healing resins into fiber reinforced composites producing self-healing fiber reinforced composites. Various chemistries have been used in the aforementioned approaches.
The aforementioned self-healing approaches address the repair, or mitigation, of crack growth and various damage conditions in materials, but have the following disadvantages: 1) Slow rates of healing; 2) Use of foreign inserts in the polymer matrix that may have detrimental effects on composite fiber performance; 3) Samples have to be held in direct contact, or under load and/or fused together under high temperature for long periods of time; 4) Do not address damage incurred by ballistic or hypervelocity impacts; and/or 5) May not be considered a structural load bearing material.
Materials that are capable of puncture healing upon impact show great promise for space exploration applications wherein an internal breach caused by micrometeoroid impacts which would normally be considered catastrophic would now be self-contained. This type of material also provides a cross-cutting route for improved damage tolerance in load bearing structures and a means of self-mitigation or self-reliability in respect to overall vehicle health and aircraft durability. In puncture healing materials, healing is triggered by the ballistic or damage event. (Ballistics tests are used to simulate micro-meteoroid damage in lab tests). The force of the bullet on the material and the materials response to the bullet (viscoelastic properties) activates healing in these materials. Polymers such as DuPont's Surlyn®, Dow's Affinity™ EG8200G, and INEO's Barex™ 210 IN (PBG) have demonstrated healing capability following penetration of fast moving projectiles—velocities that range from 9 mm bullets shot from a gun (˜300 m/sec) to close to micrometeoroid debris velocities of 3-5 km/sec. Unlike other self-healing methodologies described above, these materials inherently self-heal in microseconds due to their molecular design. For example, Surlyn® is an ionomer that contains ionic groups at low concentrations (<15 mol %) along the polymer backbone. In the presence of oppositely charged ions, these ionic groups form aggregates that can be activated by external stimuli such as temperature or ultraviolet irradiation. Surlyn®, undergoes puncture reversal (self-healing) following high velocity ballistic penetration (300 m/s-5 km/sec). The heat generated from the damage event triggers self-healing in this material. However, DuPont's Surlyn®, is not considered a load bearing material and INEO's Barex 210 IN is not puncture healing at temperatures lower than 50° C. These materials were not originally designed to be self-healing. However, their puncture-healing behavior is a consequence of the combination of viscoelastic properties under the conditions induced by projectile penetration.